Leeks

Unfortunately my leek crop has started to distort and when you open them up they have a small brown worm in them and brown marks all through the leek What is attacking the leek and is there anything I can do about it.

I had leak moth in my leeks and onions earlier in the year, i decided to harvest early while they were still only affecting the top growth. The caterpillars where off white with brown head as Bob has mentioned.